The present invention is directed to a system for reducing banding and color plane registration errors in an image forming device and, more particularly, to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums.
To print in color, electronic data representing a desired print image is initially separated into four distinct color planes; one cyan, one magenta, one yellow, and one black. Single pass full color laser printers generally include a sequence of developer stations each responsible for producing one color plane image. As a sheet of paper or other suitable media passes through the first developer station, the first color plane of the separated print image is applied. The full color print image is formed as the paper passes through the other three developer stations with each of the remaining color plane images being superimposed over the first.
Each developer station includes an insulating photoconducting material usually placed on a drum and a light source such as a scanning laser. Repeatedly scanning a beam across the drum in a series of precise lines, the scanning laser creates a latent image corresponding to one color plane on the drum""s surface by selectively exposing areas of the photoconductor drum to light. A difference in electrostatic charge density is created between the areas on the drum exposed and not exposed to light. The visible image is developed by electrostatic toners. The photoconductor drum may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. The toners are selectively attracted to the portions of the photoconducting surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor drum and the toner. A transfer roller is given an electrostatic charge opposite that of the toner and is rotated close to the photoconductor drum. The transfer roller pulls the toner from the surface of the photoconductor drum onto a sheet of paper or other print media in the pattern of the color plane image developed from the photoconducting surface. The full color image is produced as each color plane image is transferred and fused to the media sheet.
Since laser printers are designed to run very quickly, problems can arise with even slight variations in photoconductor drum speeds. Variations in photoconductor drum speed appear on the printed page as increased or decreased spacing between lines and visually appear as xe2x80x9cbands.xe2x80x9d Banding can be a particularly severe problem for laser printers when printing full color images such as photographs. In addition to banding, varying photoconductor drum speeds cause color plane registration errors. To produce accurate color prints, each successive color plane image must be precisely aligned and superimposed over the prior color plane. Color plane registration errors exceeding as little as about fifty microns, for example, produces a detectable degradation in print quality.
The principle cause of varying drum speeds is gear noise. Photoconductor drums are generally driven by a stepper motor or a brushless DC motor in connection with a gear array. Gear noise results from imperfect spacing of gear teeth, variances in flexing of gear teeth as forces are transferred from one gear to the next, and other intrinsic variations in gear force transfer. The stepper motor can also contribute to the problem because, as it drives the gear array in a laser printer, it may have slight variations in angular velocity due to the multiple magnet positions for each step.
Past solutions to banding and color plane registration errors include providing helical gears or gears made of better materials or with greater precision. These gears add significantly more expense to the final product. Solutions specific to correct banding include providing sensors to detect undesired variations in the photoconductor drum speed and additional circuitry directed to compensate the modulation of the laser accordingly. Solutions specific to registration errors include sensing variations in the speeds of the photoconductor drums and correspondingly adjusting the timing of the scanning lasers to correct the placement of each color plane image. Unfortunately, each of these solutions, requiring additional circuitry, sensing capabilities, and precision components and machining, substantially increase the manufacturing costs of an image forming device.
The present invention is directed to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. For single pass color printers in which the different color planes are developed using a series of photoconductor drums, a substantially flat movable belt is positioned adjacent to and extends across each of the drums. The belt simultaneously engages each drum so that movement of the belt past the drums causes the drums to rotate together as the belt carries media sheets past the rotating drums. In one preferred version of the invention, grit applied to the ends of the drum(s) and/or to the edge of the belt is used to transfer driving force from the belt to the drums. Conventional single pass color printers that use a paper transport belt to carry paper through the developer stations can be readily adapted to use the invention. It is expected that the invention will provide a cost effective alternative to conventional methods for reducing banding and registration errors.